The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technologies or the background thereof. The disclosure of all references cited herein are incorporated by reference.
Gases form part of many industrial chemical reactions, and for control of those reactions, gas flow rates must be controlled. Gas flow control differs from liquid flow control in that liquid quantities can be monitored using paddle wheels and other such mechanical volumetric displacement and velocity-measuring devices, whereas in a compressible gas the number of reactant molecules in a given volume rises as its pressure rises or as its temperature falls. Thus accurate control of gaseous chemical reactants requires some means of monitoring the flow rate of the number of gas molecules, rather than simply monitoring gas volume movements.
Many industrial processes require precise and reliable control of such molecular flow rates of gas into a reaction volume, and this is often a critical requirement. For example the manufacture of computer chips requires exacting control of reactive gases used for precise etching and deposition of fine structures on substrate surfaces. The flow rate of gas may change due to system drift, operating temperature, orientation change, line fouling, incorrect inlet pressure or defective valve operation. If such a change is not detected in time, the computer chip substrate wafers may be destroyed at a rate of one per minute until the change is discovered. Since such substrate wafers contain hundreds or thousands of computer chips, such errors quickly become extremely costly. Other uses for gas flow controllers are in combustion controls and testers, and for biological/pharmaceutical product manufacture. Gas flow control—specifically, the flow rate of gas molecules into a reaction chamber—is thus fundamental to many processes requiring precise dosing of chemical constituents.
The molecular flow rate of a gas may be measured as a “mass flow” of the gas, the mass of a volume of gas being directly proportional to the number of molecules in that volume. Some calibration standards thus use “gravimetric” techniques to monitor the rate of change of chamber weight with time as gas flows to or from the chamber.
A yet further way of describing the number of gas molecules in a volume, hence the molecular flow rate, is through a volumetric reference to some “standard” gas condition, which is typically the freezing temperature of water and sea-level atmospheric pressure. Such references to “standard” conditions are most accurate for gases which obey the Ideal Gas Law, PV=nkBT, where P is the gas pressure, V the gas volume, n the number of molecules in that volume, kB is Boltzmann's constant, and T the absolute temperature of the gas. The number of molecules in a volume of gas at some other pressure and temperature is then obtained by scaling that volume to what its volume would be under “standard” conditions. In such a case, one can describe the molecular gas flow rate in units of “standard cubic centimeters per minute”, “standard liters per second”, and so on. If the gas behavior approximates that of an ideal gas, which conveniently applies to most gases, this “standard” volume measure is proportional to the number of molecules flowing past each second, hence is proportional to the mass flow.
Meters and controllers in common use typically display in volumetric units, but are termed “Mass Flow” Controllers (“MFCs”) or Mass Flow Meters, reflecting their effective measurement of molecular flow rate. They are typically also calibrated volumetrically, with gravimetric calibration used in manufacturer checks.
Thermal sensing of gas mass flow is a popular industry method. Gas flows through a centrally heated tube and induces lower inlet and higher outlet temperatures, so the temperature difference between tube inlet and outlet is a good measure of the number of heat-transporting molecules flowing in a given time, as long as the tube is sufficiently thin and long such that most molecules participate. Such thermal sensors over time have been enhanced with software and hardware additions to address their limitations. Different molecules yield differing flow calibrations (U.S. Pat. No. 8,010,303 (Wang et al)), as do low inlet pressures, where gas thermal conduction and flow properties depart from those for high-pressure ‘viscous’ or ‘laminar’ flow. The thermal sensor tube output signal zero and readout varies with device rotation, for example from horizontal to vertical. Thermal and electronic zero drift also yield a lower limit to reliable flow readout, as do hysteresis effects at low flows due to baseplate and other heating history effects. With time, sensor tube sensitivity can vary, requiring recalibration of the sensor by either removal to a calibration stand or using added in-line calibration equipment described below. The readout in addition requires linearity adjustments. Since the small and thin sensor tubes can measure only low flows, typical flow sensing relies on ‘laminar flow’ bypass elements placed in parallel with the sensor tube which provide a known pressure drop, again with linearity deviations (U.S. Pat. No. 7,107,834 (Meneghini et al)). Both the sensor and laminar flow elements may clog due to fouling gases, thus yielding incorrect readouts (U.S. Pat. No. 7,243,538 (Ramsesh)). Upstream pressure fluctuation causes readout transients. An upstream pressure sensor permits readout compensations for these transients (U.S. Pat. No. 8,265,795 (Takahashi et al); U.S. Pat. No. 7,424,346 and 8,150,553 (Shajii et al)). With careful adjustments this compensation for upstream pressure changes can yield approximately constant flow readouts even in the presence of pressure jumps (Brooks Instrument Data Sheet, DS-TMF-GF135-MFC-eng, GF Series GF100/GF120/GF125 (2013)).
Gas flow control using the above thermal sensing employs a downstream proportioning valve, controlled to bring the flow readout equal to the desired flow setpoint. To check flow accuracy, a second valve may be closed on a known upstream volume with an attached pressure sensor. The rate of upstream pressure fall in that known volume yields a flow measurement for comparison with the controller readout. Such methods are described in U.S. Pat. No. 7,412,986 B2 (Tison et al); U.S. Pat. Nos. 6,363,958 B1 and 6,450,200 (Ollivier), and Brooks Instrument Data Sheet DS-TMF-GF135-MFC-eng, GF Series GF100/GF120/GF125 (2013). Since both the pressure decay and restoration of flow through the upstream valve induce upstream pressure changes with consequent possible flow variations, such check calibration is not normally done during critical process steps.
Pressure-based flow control is an open-loop control alternative to the above closed-loop sensor and control valve approach. A known pressure drop is applied across a known restrictor to yield a calculable flow rate. In its simplest form the outlet pressure is sufficiently low that it can be ignored in comparison with the upstream pressure, so that only one pressure must be monitored and controlled by an upstream valve and pressure sensor. Long capillary tube restrictors exhibit gas flow rates proportional to the square of the upstream pressure when discharging into vacuum (C. M. Horwitz, “Simple Calibrated Gas Feed System”, Rev. Sci. Instrum. Vol. 50, no. 5, May 1979, pp 652-654). Choked-flow (ie, supersonic) orifices typically exhibit flow rates linearly proportional to the upstream pressure when discharging into pressures between vacuum to approximately half of the upstream pressure; see Brown, Robert L. and Schwartz, James M. Pressure based mass flow control for ion implant SDS applications, Proceedings of International Conference on Ion Implantation Technology pp. 369-372, Vol. 1 (1998); J. J. Sullivan, S. Schaffer, and R. P. Jacobs, Mass flow measurement and control of low vapor pressure sources, J. Vac. Sci. Technol. A 7, (3), pp 2387-2393 (1989); U.S. Pat. No. 6,631,334 (Grosshart) and U.S. Pat. No. 5,868,159 (Loan et al). In these controllers flow rate is simply calculated and is not monitored.
Confirmation of gas flow rate is thus required for both thermal and pressure-based controllers. Upstream methods of flow testing are described in the aforementioned Tison and Ollivier patents and the Brooks Instrument datasheet; and downstream volumetric or gravimetric methods are described in U.S. Pat. No. 7,975,558 (Lee et al) and U.S. Pat. No. 6,216,726 (Brown et al). A gas control method which eliminates the need for complex and expensive controllers and calibration systems would therefore be highly desirable. An auto-calibrating and auto-adjusting controller which can report to users its status in real time while maintaining the required flow would be a further desirable feature. A gas control method which does not require recalibration when changing between gases would simplify device stocking, calibration checking, and test, and would be a further advantage.